The terms “biofuel” and “biofuel” refer to fatty acid monoesters made from oils comprising triacylglycerol lipids. Generally, biofuels are produced from oils such as soybean oil, linseed oil, sunflower oil, castor oil, corn oil, canola oil, rapeseed oil, palm kernel oil, cottonseed oil, peanut oil, coconut oil, palm oil, tung oil, and safflower oil, as well as derivatives thereof.
Several different methods are currently being used for producing biofuel from such starting materials. One such method involves hydrolyzing fatty acids from the triacylglycerol to form glycerol and free fatty acids. The free fatty acids are separated from the glycerol and reacted with a monohydric alcohol in the presence of a liquid phase acid catalyst to form the fatty acid monoester and water. The water is then removed to produce biofuel. This acid hydrolysis followed by esterification is chemically efficient, but the overall process requires two different chemical reactions and two different separations steps and is therefore not economically efficient.
Another method that can be used to make biofuel employs direct alcoholytic transesterification of the triacylglycerol with the monohydric alcohol to form the fatty acid monoester and glycerol, followed by separation of the fatty acid monoester from the glycerol. This method can be more efficient than the previous method because only a single reaction and single separation step need to be performed. However, maximum efficiency of the reaction typically requires the presence of a catalyst.
The most widely used catalyst is a liquid hydroxide, typically sodium hydroxide or potassium hydroxide dissolved in methanol to generate a methoxide ion, which is highly reactive. The amount of hydroxide catalyst required for efficient alcoholytic transesterification is at least 0.75% wt/vol hydroxide to methanol, and more typically about 1% to 5% wt/vol. Even at these amounts however, the reaction typically converts only about 80% of the triacylglycerols into fatty acid monoesters before reaching an equilibrium.
An additional method for producing biofuel involves the enzymatic transesterification of the triacylglycerol. For example, U.S. Pat. No. 5,713,965, the disclosure of which is incorporated herein in its entirety, describes a method that utilizes lipases to transesterify triglyceride-containing substances and to esterify free fatty acids to alkyl esters using short chain alcohols. Employing enzymatic transesterification for industrial scale production of biofuel suffers from several shortcomings, not the least of which is that the methanol that is employed as the acyl acceptor in the transesterification reaction inactivates the lipase, resulting in a reduced enzyme activity over time. Additionally, the process is limited by the activity of the enzyme itself.
What are needed, then, are new methods for increasing the efficiency of enzymatic transesterification of renewable oils to produce biofuel.